Polar transmitters have been utilized in mobile devices for their flexibility and performance benefits over their traditional rectangular format counterparts. Polar transmitters have been proven to be amenable to implementation in low-voltage nano-scale CMOS technology. FIG. 1 shows a conceptual block diagram of a conventional polar transmitter 100, which may include a modulator 105, a rectangular to polar converter 110, a differentiator 115, an adder 120, a voltage controlled oscillator (VCO) 125, a mixer 130, and a power amplifier 135.
The polar transmitter 100 may receive symbols as an input signal having a Cartesian representation (i.e., a complex signal with in-phase (I) and quadrature (Q) components), which may also be denoted herein as a “rectangular-form.” The input signal may be converted to a polar representation (also denoted herein as a “polar-form”) using the rectangular to polar converter 110. The rectangular to polar converter 110 produces two signal components, an envelope (i.e., magnitude) component and a phase component. The phase component may be converted to frequency by a digital frequency converter shown as the time differentiator 115, and then offset (i.e., up-converted) to a carrier frequency fc using the adder 120. The upconverted frequency may be used to drive the voltage controlled oscillator 125 which generates a modulated sinusoidal signal. The envelope signal from the rectangular to polar converter 110 can be multiplied by the modulated sinusoid signal to provide an input for the power amplifier 135. The power amplifier 135 may amplify this signal for transmission through an antenna (not shown).
Traditionally, polar transmitters have been successfully used in devices utilizing narrowband modulation standards, such as the 2G cellular or EDGE standard. However, polar transmitters face challenges when used in standards based on wideband modulation techniques, such as 3G (e.g., WCDMA), 4G (e.g., 3GPP LTE, WiMAX), and the like. The non-linear polar to rectangular transformation may result in much larger dynamic range and signal bandwidths as the complex signal trajectory approaches or crosses the constellation origin in the I-Q plane.
A representation of the I/Q plane 200 is provided in the diagram shown in FIG. 2, where two separate signal trajectories 205, 210 are shown. Each signal trajectory may represent a locus of samples provided by the modulator 105 over some period of time. It can be seen that in signal trajectory 205, the differential phase Δθ1 is larger than the differential phase Δθ2 corresponding to signal trajectory 210, because signal trajectory 210 is further displaced from the origin. The large differential phase values may result in high sample-to-sample frequencies that may exceed the linear region of the VCOs used in the polar transmitter 100, which may cause unacceptable distortions in the transmitted signal.
Also, the envelope component output from the rectangular to polar converter 110 may exhibit large peaks, thus having a large Peak to Average Power Ratio (PAPR). Signals having a large PAPR may also present difficulties for power amplifiers, as such peaks may drive the amplifiers into non-linear operation, or may event result in saturation such as clipping.
Conventional approaches for meeting the aforementioned challenges may include a “hole punching” solution for reducing the amplitude and phase bandwidths. Such techniques may alter the signal trajectory such that it avoids a defined proximity about the constellation origin. However, these conventional techniques may introduce a variety of non-linear distortions in the polar transmitter's signal path and/or introduce other distortions that can increase the Error Vector Magnitude (EVM) of the amplified signal.